Hitherto, it has been practice to fabricate domed structures having cellular walls, such as pressure bulk-heads in aircraft, from suitably-shaped individual component parts that are joined together, e.g. by rivets. However, such a practice is complicated because each part must be individually shaped and the shaped components must be exactly positioned before riveting; furthermore, the presence of the rivets adds to the weight of the structure with is disadvantageous, particularly in aerospace applications.
Prior art has suggested methods of making shallow domed structures of regular shape (see U.S. Pat. Nos. 4,833,768 and 3,024,525 discussed below); however, no teaching is contained in the prior art for making deep-domed structures especially those of an irregular shape.
U.S. Pat. No. 4,833,768 describes a method of forming a cellular domed structure from two or more sheets. In this process the sheets are joined together to form a stack and placed between two moulds, one of convex form and the other of concave form; the moulds are then slowly brought together so that the convex mould presses against the stack of sheets to form the stack into a domed shape; the pressing of the stack of sheets continues until the mould is closed (this technique is known as "creep-formation"); then gas is injected into the space between the sheets to inflate the stack of sheets and superplastically form them into a shallow cellular domed structure. The method described cannot be used to form a deep-domed structure or a structure having a shape that is not a smooth dome because creep forming cannot stretch the sheet sufficiently to form such structures without necking or fracturing the metal, that is to say creep-forming can achieve an elongation only of the order of 20%, which does not allow formation of a deep-domed structure or a structure having a shape that is significantly different from a smooth dome.
U.S. Pat. No. 3,024,525 describes a method of making a multi-walled shallow-domed structure of circular-symmetric shape, e.g. for use as parabolic radar reflector dishes. The structure is made by joining two sheets together, slowly pulling the sheets over a domed die to creep form the sheets into a domed shape that is complementary to the shape of the domed die; gas is then injected into the space between the sheets to force the outer sheet away from the inner sheet. The outer sheet provides a strengthening backing sheet to the inner sheet. Such a method is not suitable for forming deep-domed structures nor domed structures of irregular shape because, as discussed above, creep forming can only achieve limited stretching of the sheets concerned.
It is well known that certain metals have superplastic characteristics, i.e. a composition and micro-structure such that when heated to within an appropriate temperature range and when deformed at a strain rate within an appropriate range they exhibit the flow characteristics of a viscous fluid. With such metals, large deformations (e.g. in excess of 100-200% and typically of order of 1000%) are possible without fracture. Diffusion bonding is also a well-known process by which a metallurgical bond is formed by the application of heat and pressure to metallic pieces held in close contact for a specific length of time. Bonding is thought to occur by the movement of atoms across adjacent faces of the pieces. The process allows metals to be joined without significantly changing their physical or metallurgical properties. The temperature ranges at which superplasticity and diffusion bonding occur may or may not be the same depending upon the material joined. It is also known to combine diffusion bonding and superplastic forming in a final shape by superplastic forming. It is known from U.D. Patent No. 1,378,421 to form an inflatable envelope by metallurgically bonding together peripheral regions of two substantially flat sheet member of metal having superplastic characteristics, heating the envelope to within the temperature range for superplasticity of the metallic alloy, and applying a differential pressure between the interior and the exterior of the envelope while it is within the said temperature range such that the envelope expands as a balloon. The end product is a spherical container suitable for use as a pressure vessel and not domed structure having walls of a cellular structure.
In U.K. Patent 1,429,054 a method of forming a stiffened panel is taught in which an interior sheet is placed between two face sheets and the inner faces of each of the face sheets are attached directly to the respectively adjacent faces of the interior sheet by metallurigically bonded regions. At least the interior sheet is of superplastic material. The sheets are welded around their assembled peripheries to form a sealed envelope which is fed internally with a pressurized inert gas such that when the interior sheet is superplastic the outer sheets are moved apart by a predetermined amount to affect corrugation of the interior sheet thus effectively achieving a cellular structure of `zig-zag` construct. The final panel is not domed.
In yet a further arrangement, as exemplified by U.S. Pat. Nos. 4,217,397 and 4,351,470, twin interior sheets forming a sealed envelope and being joined to each other by metallurigically bonded regions are supplied with inert gas under pressure, thereby causing the sheets to move away from each other within a limiting fixture, which may include face sheets, the envelope being expanded against the face sheets to form a series of cavities between the sheets, the said cavities being preferably of substantially rectangular form. The process is generally applied to form flat panels but the patent specification says that it is possible to use face sheets that are not flat. However, the forming of such non-flat sheets is complex.
U.S. Pat. No. 4,526,312 teaches a method of making metallic sandwich structure by superplastic forming and diffusion bonding using a three sheet stack but whereas in the previous examples of prior art the expanded assembly remains substantially flat, in this example the sandwich structure can be used to achieve single curvature configuration, that is to say the structure has a straight ridge along an external face. The flat stack is placed in a mould and simultaneously or sequentially formed to a desired curvature; it is then trimmed and the trimmed stack is placed in a further mould defining the boundaries of the required component form and then formed into the expanded sandwich structure. In the arrangement as taught the corrugations or `zig-zag` stiffeners lie transversely to the radius of curvature parallel, i.e. the stiffeners lie parallel to the ridge.
Although the referenced prior art teaches a deep drawn pressure vessel and alternative arrangements of cellular or sandwich structures using superplastic forming and/or diffusion bonding techniques, there is, as far as we are aware, no known method of construction in the art for producing a deep domed cellular structure or a domed structure of irregular shape, for example a pressure bulkhead in an aircraft fuselage and in particular neither U.S. Pat. No. 4,833,768 now U.S. Pat. No. 3,024,525 discloses a process capable of making such a structure.
It is the object of the present invention to provide a method of making a deep-domed structure.